A typical metallurgical furnace is a container having sidewalls with a multi-layer construction. The outer layer is typically a steel shell provided for structural support. The inner layer includes a refractory lining, constructed from one or more layers of refractory bricks, that is provided to shield the outer steel shell from molten materials and aggressive chemicals inside the furnace. In some furnaces, a cooling layer is also provided between the outer steel shell and the refractory lining to prevent excessive heat transfer from the refractory lining to the outer steel shell. In some furnace designs, the layers of brick and/or cooling elements are set in place with a soft sand-like material that solidifies during the operation of the furnace.
During the operation of a metallurgical furnace, the refractory lining can be deteriorated by mechanical and thermal stress in addition to chemical degradation resulting in a loss of overall refractory lining thickness. As the refractory lining deteriorates molten materials and aggressive chemicals penetrate into widening spaces in and/or between refractory bricks leading to delamination (i.e. separation) of the layers in the refractory lining. Such delamination can exert expansive stresses on the outer steel shell, and may cause local deformation of the steel shell. Deterioration of the refractory lining can also lead to structural failures that may cause the outer steel shell to be exposed to molten materials and aggressive chemicals inside the furnace.
Moreover, if the molten materials and aggressive chemicals reach the outer steel shell there is an imminent risk of severe injury to personnel working near the furnace, because the outer steel shell is typically not capable of reliably holding back the molten materials and aggressive chemicals inside the furnace. Loss of heat transferability and conductivity are also known to occur as a result of the deterioration of the refractory lining, both of which may contribute to deformation and failure of the steel shell.
Another mode of refractory lining deterioration, common in furnaces that include water-cooled elements, is hydration of the refractory lining. Under certain temperatures, water that has leaked from a cooling element can react with the refractory bricks causing expedited deterioration of the refractory lining. In particular, magnesia (MgO) based refractory bricks are susceptible to this mode of failure. Expansion of the refractory lining caused by hydration can exert increased expansive pressures on the steel shell, and may contribute to local deformation of the shell.
It is desirable to monitor the structural integrity of the furnace, and particularly the outer steel shell, in order to help predict the timing and location of shell ruptures. Making a reliable and accurate assessment of the condition of the steel shell is difficult to do without first emptying the furnace and shutting down the industrial process in which the furnace is involved. Shutting down a metallurgical furnace for routine inspection is costly and operators try to make use of inspection methods that can be employed while the furnace is operating. However, the hostile working environment in which a furnace is typically used skew the measurements made. For example, extremely high temperatures in the furnaces, vibrations, ambient noise, dust, and electrical and mechanical hazards are known to distort the structural integrity measurements generated by the previously known inspection methods. Conventional deformation monitoring tools, such as strain gauges, can be useful, but can only measure deformation in the vicinity in which they are installed. Further, conventional strain gauges are generally unable to differentiate between elastic and plastic deformation of the outer shell.
A systematic method monitoring the structural integrity of the steel shell and identifying the beginning cracks and other structural defects in real-time has not been developed. As a result, operators are forced to shut down and cool furnaces in order to check the shell integrity from time-to-time, and may have little or no warning of an impending shell rupture which can result in the leakage of molten metal from the furnace into the surrounding environment (a run-out).